Gas, oily matters (tar), solid hydrocarbon (char) and ash are produced in gasification of such hydrocarbons as coal, vacuum residual oil, dry asphalt, tar sand, oil shale and mixtures of them. The gasification temperature is so high (600.degree.-1500.degree. C.) that it is necessary to improve thermal efficiency of the whole gasification plant by effectively recovering sensible heat of products. In this process, it is effective to recover heat from gas with the highest sensible heat (containing oily matter) among said products. Heat recovery method by indirect heat exchange with a fluidized bed type has been developed for this purpose. According to the method, sensible heat of gas is extracted in the form of high temperature steam from the heat transfer tube by introducing said gas into the fluidized bed with heat transfer tube and carrying out heat exchange through fluidized particles. In this case a part of oily matters are cooled to cokes. But cokes are deposited on the surfaces of actively moving fluidized particles, so there is no possibility of plugging of equipments by coking as occurred in a shell and tube type heat exchanger method. If cokes deposit on the surface of heat transfer, they are scraped continually off by actively moving fluidized particles and do not accumulate on the surface of the tube and heat transfer efficiency is constantly kept at the initially established level.
It is desirable for the heat recovery method by heat exchange system with use of the fluidized bed to lower the operating temperature of the fluidized bed heat exchanger in order to increase heat recovery efficiency and recovery quantity from oily matter (cokes quantity), but if the operation temperature lowers, the temperature of the recovered steam lowers, quality of recovered heat degrades and oily matters do not deposit at the coke state but liquid state on the fluidized particles and movement of fluidizing particles are hindered. In addition, in order to regenerate particles by removing cokes accumulated on particles, regenerating processes such as a fluidized bed boiler or a combustion furnace should be added, and equipments become complex and require higher cost and the operation and the control get complicated.
Because of the above facts, the lowest operating temperature of the fluidized bed heat exchanger is set about 400.degree.-450.degree. C. conventionally, and it has been tried to use the identical particles with those used in the gasifier as fluidized particles and to regenerate particles by the gasifier itself. In these methods, however, heat recovery efficiency is not always satisfactory and it is generally difficult to apply the methods to various types of processes.
FIG. 1 shows the results of the experiments forming the basis of the invention. In this experiment, high temperature product gas produced by gasifying the mixture of coal and residual oil by vacuum distilation with oxygen and steam is passed through the fluidized bed with sand and the increase of the sand weight is measured with the passage of time. In FIG. 1, the time elapsed T(h) from the start of passing the product gas is marked on the axis of abscissas and the weight increase W (kg) calculated from the initial weight of the filling-up matters in the bed and ultimate analysis of fluidized particles at the each point of time is marked on the axis of ordinates. The temperature of thermal cracking zone in the gasifier is 750.degree.-780.degree. C. and temperature control of sand fluidized bed was done by an electric heater.
The following facts are known from the experimental results in FIG. 1.
(1) Cokes deposit on sands steadily with the passage of time, but the increase ratio of deposit amount drops when one hour passed as for instancce, the curve over 450.degree. C. shows. This is because deposited cokes are peeled off by the rubbing of particles and rubbed cokes are dispersed with gas. PA1 (2) The fluidized bed gets wet at low temperature within relatively short time and the fluidization is hindered (oblique-lined portion). This phenomenon is not seen above 450.degree. C. PA1 (3) The deposit amount (to be referred to as critical deposit amount in the following) which causes a fluidization hindrance is nearly definite at any temperature.
It is known from the above facts that the balance between the amount of fed oily matters and coking rate affects the coking amount in the fluidized bed. If the feeding amount exceeds the coking rate, liquid matters deposit in the bed and hinders the fluidization, but, below a certain deposit amount, the fluidization is not affected. Therefore stable operation can be obtained at low temperature of about 300.degree. C. with deposit amount below the critical deposit amount in the fluidized bed.
The control of deposit amount below the critical point is obtained by regenerating the particles in the bed. As shown by the curving line of 300.degree. C. in FIG. 1, regenerating should be done within 12 minutes and residence time of the particles should be extremely short. It is proposed to circulate the fluidized bed particles between the heat recovery zone and regeneration zone (for example, Published unexamined patent application Showa 51-127101, Japanese official bulletin), but by this method the large amount of particle must be circulated and this increases the power cost and decreases operativity.